Page 59 - General Biochemistry

3. Niacin (nicotinic acid)

• The biologically active co-enzyme forms are nicotinamide adenine
dinucleotide (NAD+) and its phosphorylated derivative, nicotinamide adenine
dinucleotide phosphate (NADP+).

• NAD+ and NADP+ serve as co-enzymes in oxidation-reduction reactions in
which they undergo reduction to NADH and NADPH, respectively. N.B. NAD+
is used mainly for oxidative catabolism while NADPH is used mainly for
reductive biosynthesis. NADPH is also a co-enzyme for cytochrome P450
reductase responsible for drug metabolism.

• Deficiency of niacin causes pellagra, a disease involving the skin, GIT and
CNS.

• The symptoms of pellagra are dermatitis, diarrhea and dementia (3 Ds); if
untreated, death occurs.

4. Pyridoxine (vitamin B6)

• Vitamin B6 is a collective term for pyridoxine, pyridoxal, and pyridoxamine.
• The biologically active co-enzyme form is pyridoxal phosphate. It acts as a co-

enzyme for enzymes that catalyze reactions of amino acids such as
transamination, deamination & decarboxylation.

• Dietary deficiency in pyridoxine is rare; however, drug-induced deficiency can
occur with isoniazid (antituberculous drug). Isoniazid can induce vitamin B6
deficiency by forming an inactive derivative with pyridoxal phosphate; dietary
supplementation with vitamin B6 is thus recommended as adjunct to treatment.

5. Biotin

• Biotin is a co-enzyme in carboxylation reactions, in which it serves as a carrier
of activated carbon dioxide.

• Biotin deficiency does not occur naturally because the vitamin is widely
distributed in food and is supplied by intestinal bacteria. However, the addition
of high amounts of raw egg white to the diet tightly binds biotin (through
complex with avidin in egg white) preventing its absorption.

58